The present invention relates in general to electrophotographic devices, and more particularly, to systems and methods for distributing a line of image data across at least two scan lines in a scanning laser beam electrophotographic device.
In electrophotography, an imaging system forms a latent image by exposing select portions of an electrostatically charged photoconductive surface to laser light. Essentially, the density of the electrostatic charge on the photoconductive surface is altered in areas exposed to the laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam. The toner pattern is subsequently transferred from the photoconductive surface to the surface of a print substrate, such as paper, which has been given an electrostatic charge opposite that of the toner.
A fuser assembly then applies heat and pressure to the toned substrate before the substrate is discharged from the apparatus. The applied heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the applied pressure promotes settling of the toner constituents in these voids. The toner solidifies as it cools adhering the image to the substrate.
In a typical laser scanning system, a rotating polygon mirror is used to sweep a laser beam across a photoconductive surface in a scan direction while the photoconductive surface advances in a process direction that is orthogonal to the scan direction. A scan line is written each time a new facet of the polygon mirror intercepts the laser beam. Correspondingly, bitmap image data is communicated to the laser source such that one scan line writes a corresponding line of bitmap image data to the photoconductive surface. The polygon mirror speed is synchronized with the advancement of the photoconductive surface so as to achieve a desired image resolution, typically expressed in dots per inch (dpi), at a given image transfer rate, typically expressed in pages per minute (ppm).
Changes in scanning resolution and/or in the image transfer rate can be accomplished by adjusting the process speed of the photoconductive surface. For example, slowing the photoconductive surface to one half of the full speed image transfer rate without changing the scanning mirror speed can provide double scan line addressability which, ideally, can improve the quality of the image printed on the medium. Additionally, by operating the photoconductive surface and optionally, the scanning mirror, at half speed, greater time is available for fusing operations because the print medium is moving through the device at a slower speed. Relatively longer fusing times are desirable for example, when the print medium is relatively thick or where transparencies are used. As another example, print resolution maybe changed by modifying the scanning mirror speed while printing at the same image transfer rate.
Modifying the process speed and/or the polygon mirror speed also often requires a drastic change in laser power to compensate for the changing printing characteristics. For example, to operate satisfactorily at one half of the full speed (normal) image transfer rate, and to maintain double line addressability, e.g., by maintaining the normal scanning mirror speed, the laser power needs to be reduced by one half of the full speed laser power so as to maintain output image consistency between full speed and half speed image transfer modes of printing. Unfortunately, the operating range of a typical laser diode may not allow such drastic changes in laser output power. As such, the prior art has attempted to reduce laser power output by using pulse width modulation of a full power laser beam such that the power output by the laser is reduced by one half. However, pulse width modulating a laser beam increases the complexity of the laser diode driver circuitry. Moreover, changing the duty cycle of a laser beam affects the “turn on” and “turn off” characteristics of the laser, which may affect overall consistency and print quality.